Botulinum Neurotoxins are the most lethal substances known, and depending on the serotype, the estimated human lethal dose ranges from 1 to 3 ng toxin per kg body weight. Due to their ease of purification and high potency, BoNTs pose a real and potential threat for use as bioweapons. Further, BoNTs are increasing used for cosmetic and therapeutic purposes. Currently, the majority of BoNT detection and potency measurements completed in government and industrial laboratories are done using animal-based assays that suffer from high costs and poor accuracy.
The primary advantage of using cell-based assays (CBAs) for BoNT potency measurements is that CBA systems closely mimic the individual physiologic steps that occur in neurons during intoxication. BoNTs are zinc-dependent endopeptidases composed of a heavy chain, responsible for neuron-specific receptor binding and cell entry, and a catalytic light chain responsible for synaptic protein cleavage. Upon entry into neurons, the BoNTs specifically disrupt the protein machinery responsible for fusion of synaptic vesicles with the plasma membrane, thereby inhibiting neurotransmitter release into the post-synaptic junction. Because all physiologic steps must be accounted for in CBAs, most assays fail to meet the sensitivity requirements of BoNT activity measurement.
The employment of established, stable model cell lines for the detection of BoNTs have recently been described, however, past CBAs fail to meet the sensitivity requirements for the quantification of pharmaceutical preparations of BoNT or the detection of BoNT clinical samples. Presumably, current established model cell lines lack critical neuronal characteristics required for efficient BoNT uptake. A recent solution to increasing BoNT sensitivity is the use of mouse embryonic stem cells (mESCs). Because these cells can be terminally differentiated into neurons they are more sensitive to BoNT treatment. A major drawback to this method, though, is the multiple weeks required to fully differentiate mESCs.
Thus, there is an urgent need to development cell based assays for assessing and quantifying the potency of BoNT-containing samples for bioweapon defense, food home illness, and therapeutic purposes.